Property Update function for properties inside a collection
I can't tell whether it's a known limitation or bug, but it definately returns the reference to the CollectionProperty only.

You need a concept of one element in the collection being "active", e.g. have an IntProperty alongside and use it for a template_list() for instance. Then use that number to get the right item:

self[context.scene.ppp_index].pfloat = 1.23




There's actually a reference to the CollectionProperty item returned,
see https://developer.blender.org/T39798